The current aims of this research program all involve electron microscopy of DNA and include: 1. The reconstruction of partial denaturation maps of very large DNA molecules from the maps of random (shear) fragments by computer sorting and alignment of the maps of the fragments. 2. The construction by DNA heteroduplex methods, of a physical/genetic map of the Salmonella phage P22 using insertions of translocatable drug-resistance elements at known genetic locations and deletions with known genetic endpoints (some of which are generated by the translocatable elements) to make the correlation between physical distance and genetic location. 3. The analysis of the mechanism of excision of translocatable drug-resistance elements by heteroduplex analysis of the DNA of drug-sensitive and deletion "revertants" of bacteriophages (coliphage lambda as well as P22) carrying the various elements inserted into their genomes.